1. Field of the Invention
This invention relates to a toner for developing a static latent image and an image forming method.
2. Description of Related Art
Most of electrophotographic photoreceptors have been shifted from inorganic photoreceptors to organic photoreceptors, the latter of which exhibit advantages such as minimal pollution and ease of production, and the above organic photoreceptors (hereinafter also referred simply to as photoreceptors), employing various materials, have been developed.
In recent years, function-separating type photoreceptors in which a charge generating function and a charge transport function are performed employing different materials have become the main current. For example, widely employed are photoreceptors which are composed of a conductive support having thereon a surface layer incorporating inorganic particles (refer to Patent Document 1).
Further, when attention is paid to electrophotographic processes, latent image forming systems are divided mainly into analog image formation employing halogen lamps as a light source, and digital system image formation employing LEDs and lasers as a light source. Recently, digital system latent image forming systems are increasingly becoming the mainstream due to application to hard copy printers of personal computers, as well as ease of image processing and ease of development to composite devices in conventional copiers.
Further, in the image forming method employing a digital system, the frequency of preparation of original print images increases, whereby demand for higher image quality has been increased. In order to enhance the image quality of the above electrophotographic images, a technique has been developed in which a highly detailed latent image is formed on an organic photoreceptor employing an exposure light source of a small spot diameter, and the resulting minute dot images are converted to a toner image.
Namely, known as development methods of latent images on an organic photoreceptor are a development system (hereinafter referred to as a parallel development system) in which a developing sleeve facing the organic photoreceptor is advanced parallel to the advancing direction of the organic photoreceptor in the development region, and a development method (hereinafter referred to as a counter-development system) in which the developing sleeve is advanced in the opposite direction (refer to Patent Document 2). However, when high density dot images are formed, neither have completely overcome the above drawbacks.
In the development system in which a developing sleeve facing the photoreceptor is advanced parallel to the advancing direction of the photoreceptor, developability of the periphery of images suffer, resulting in tendency of insufficient density, whereby high contrast photographic images tend to suffer from the degradation in image quality.
On the other hand, a development system, in which the developing sleeve is advanced in the opposite direction, enables the formation of high density dot mages due to high developability, but fogging and insufficient density at the leading edge tend to occur.
It has been found that such problematic phenomena are not minimized only by improvement of developers and are enhanced or minimized depending on characteristics of the photoreceptors.
Further, it is assumed that the above phenomena relate to the contrast of electrostatic latent images formed on an organic photoreceptor and the formation of reversely charged toner due to friction between the organic photoreceptor and the developer.
Namely, in the counter-development system, a reversely charged toner tends to form due to contact friction with the photoreceptor. As a result, fogging and toner scattering tend to occur and a decrease in density of the leading edge portion also tends to occur, whereby it is not possible to reproduce a highly detailed electrostatic latent image as a toner image. Heretofore, it has been proposed to use a polymerization toner of which particle size distribution is narrowed in a development means (refer to Patent Document 3). However, it was found that a toner exhibiting the proposed size distribution, when employed in the counter-development system, was not capable of sufficiently retarding the formation of reversely charged toner, whereby it was not possible to reproduce highly detailed electrostatic latent image as a toner image.
Further, for preparing high density images by increasing the shielding effect of toner, a toner is known in which the shape of particles is flattened (refer to Patent Document 4). However, since the above toner composed of flattened particles easily adheres to a carrier to result in tendency of degraded developability, it has not been possible to prepare high density images.
(Patent Document 1) Japanese Patent Publication for Public Inspection (hereinafter referred to as JP-A) No. 8-262752
(Patent Document 2) JP-A No. 2001-125435
(Patent Document 3) JP-A No. 2002-244336
(Patent Document 4) JP-A No. 2003-29444